Mobile communication systems are well-known, a universal mobile telecommunications system (UMTS) being an example of such a known system. In a UMTS system, a mobile device, known as a user equipment (UE), connects to a mobile communications network infrastructure via one or more node-Bs. A dedicated physical channel is established between the user equipment and one of the node-Bs to allow data communication to take place therebetween.
Each node-B in a UMTS system supports one or more cells of radio coverage. As a user equipment roams, it is necessary for a handover of the dedicated physical channels to take place from cell-to-cell. Techniques are well-known in the art for facilitating such handover. Such techniques typically rely upon the network side of the system assessing parameters measured at the user equipment, to determine if the carrier signal strengths are such that it would be appropriate to handover to a different cell. There are various different types of handover, which can be broadly split into the categories of soft handover and hard handover. Hard handover comprises either intra-frequency hard handover or inter-frequency hard handover.
In certain types of hard handover, there is a requirement for the system frame number (SFN) timing to be maintained between the existing dedicated physical channel and the newly established dedicated physical channel after handover. In this case, the 3GPP specification does not require the decoding of the SFN of the target cell before establishment of the channel.
In hard handover, following a decision by the network side to perform a handover, based on measurement of carrier signals received at the user equipment, the node-B allocates resources to the channel on which the dedicated physical channel is to be established. Thereafter an instruction to reconfigure the dedicated physical channel is sent to the user equipment, and a synchronization procedure and physical channel establishment procedure initiated. A problem arises in that the allocation of resources to the new channel may take some time in the node-B, for example of the order of a few seconds. This delay may be because the node-B is overloaded. Thus by the time the user equipment receives the message to reconfigure the physical channel, the radio conditions may have changed to such an extent that the quality of the radio signal on the proposed new channel has deteriorated below the quality of the radio signal on the existing channel. The physical channel establishment procedure associated with the synchronization procedure may not succeed for the proposed new channel if the channel conditions have deteriorated to such an extent that radio contact can simply no longer be detected. This will be continued with until a ‘Physical Channel Establishment Failure’ is detected. The synchronization phase is controlled by a timer, the timing-out of the timer indicating that a Physical Channel Establishment Failure has occurred, due to synchronization being lost.
The timer may not time-out, however, for a notable period of time, for example 15 seconds. During this time the establishment of the new dedicated physical channel is attempted, with no prospect of success. Only when the timer times-out can the node-B initialize re-establishment of the previous dedicated physical channel. Such failure would be apparent to an end-user.
It is an aim of the invention to provide an improved technique for controlling the initialization/establishment of a new dedicated physical channel in embodiments where it is necessary to maintain system frame number timing between the new dedicated physical channel and the previous dedicated physical channel.
It is a further aim of the invention to provide an improved technique for controlling the initialization/establishment of a new dedicated physical channel in embodiments where the 3GPP specification does not require to decode the SFN of the target cell before establishment of the channel.